This “American Idol” beauty has to work to flaunt her svelte figure.Carrie Underwood tells Extra that her tummy “is an Underwood family tradition” and that her rear end “starts going in and out so I have to work really hard to have it stay where it’s supposed to be.” She says her diet has changed to exclude junk food and she tries to stay hydrated with both regular water and Vitamin Water.Carrie also gave Shape her exercise secrets in the February 2008 issue (pictured above). “We got really simple, basic stuff, because if it was complicated, I knew I wouldn’t have used it. My favorite thing is cardio. It really helps the pounds melt off, and it gives me so much more energy … Working out has really paid off on stage.”This Idol beauty is anything but idle — Carrie just released her third album, “Play On,” on Nov 3, she’s co-hosting the Country Music…